The present invention relates generally to a process for the preparation of a mercaptan addition product. More particularly, the present invention relates to a process for the preparation of a mercaptan addition product by the free-radical addition of a mercaptan to selected nonconjugated olefinic substrates having a terminal carbon-carbon double bond, the mercaptan adding across the terminal carbon-carbon double bond of the substrate. The process may be used to prepare 2-hydroxyl-4-(methylthio)butanoic acid (HMBA) (or methionine.
HMBA is the hydroxy analog of methionine, an essential amino acid commonly deficient in grains used in animal feed compositions. HMBA is a metabolite in methionine utilization, and is widely used as a methionine supplement in animal feed formulations.
One way of producing HMBA involves the Michael addition of methyl mercaptan to acrolein in the presence of an organic amine catalyst (e.g. pyridine) to produce 3-(methylthio)propanal (MMP). The MMP is then reacted with hydrogen cyanide in the presence of an organic amine catalyst (e.g., the same catalyst used in the olefin/mercaptan addition reaction) to produce 2-hydroxy-4-(methylthio)butanenitrile (HMBN). The HMBN is subsequently hydrolyzed to produce HMBA. This reaction sequence is shown below. ##STR2##
Although the above-described sequence has proven to be acceptable in the commercial preparation of HMBA, it would be highly beneficial to identify alternative synthesis routes for producing HMBA as well as methionine.